Remove the paper wrapper from 2 empty, BPA-free 28-ounce cans. Wash and dry them, then oil the bottom and sides with cooking spray. Cut two circles of parchment paper to fit the bottoms of the cans, press them on the bottom surface to lay them flat.
Whisk the dry ingredients together in a mixing bowl.
Add the molasses and buttermilk to the dry ingredients, stirring just until combined.
Divide the batter between the 2 cans, tapping them on the counter to level out the batter. Cover the top of each can with foil, secured tightly around the sides.
Put the cans in a large pot on the stove with water halfway up the cans. I weigh the cans down with small plates, to help seal the cans while the bread steams. The tighter the seal, the more effective the steaming.
Cover the pot and bring the water to a simmer on medium-high heat. Reduce the heat to low and keep it simmering for 45-60 minutes, until the breads are set on the top and sides. The bread should pull away slightly from the sides of the can. The internal temperature of the bread should be 200˚F / 93˚C.
Remove the cans from the pot. Set them on a wire rack and cool the breads in the cans. Once they reach room temperature, use a knife to loosen the bread from the sides of the can, then turn the can upside down and tap it lightly. The bread should slide out.